The present invention relates to an insulation measurement apparatus and, in particular, relates to an insulation measurement apparatus which can measure an insulation resistance accurately by using a ceramic capacitor as a flying capacitor.
Conventionally, an automobile mounts a battery for charging electric power therein and for supplying electric power to electrical equipments such as a light turning-on system and an air conditioner. Automobiles of the day depend on electric power as is no exaggeration to say that the automobiles can not work without electric power.
Further, the regulation of exhaust gas has been enhanced in view of the battle against global warming etc. Thus, some of automobile manufacturers bring hybrid cars each employing an engine and a battery as driving power sources to the market. Such a tendency having been accelerated and so many automobiles employ batteries as driving power sources.
Under such the background, the management of electric power has become more important for automobile manufacturers. In particular, in the case of mounting a high-output battery for driving, since the voltage is very high as compared with the conventional voltage, the probability of getting an electric shock is high when the insulation property is degraded. Accordingly, it has become more important to monitor the insulation state.
Although various types of techniques for determining the insulation state have been introduced, there is an insulation measurement circuit of a flying capacitor type, for example (see JP-A-2007-170983). FIG. 1 is a circuit diagram of an insulation measurement circuit 110 disclosed in JP-A-2007-170983. The insulation measurement circuit 110 is constituted by a detection circuit 120 and a determination control portion 130 and detects the insulation state of a power supply V. The detection circuit 120 includes a capacitor (flying capacitor) C11 in an electrically floated state from the ground voltage G, first to sixth resistors R11 to R16 and first to fourth switching elements SW11 to SW14. The determination control portion 130 turns the first and second switches SW11, SW12 on to form a path from the positive electrode side of the power supply V to the negative electrode side thereof via the first switch SW11, a first diode D11, the resistor R11, the capacitor C11 and the second switch SW12 to thereby set a voltage (this voltage is called “a high voltage V10”) of the power supply V to the capacitor C11. When the first and second switches SW11, SW12 are turned off and the third and fourth switches SW13, SW14 are turned on, a closed circuit is formed by the capacitor C11, a second diode D12, the second resistor R12, the third switch SW13, the sixth resistor R16, the third resistor R13, the fourth resistor R14, the fourth switch SW14. Thus, a voltage divided by the second resistor R12, the third resistor R13 and the fourth resistor R14, that is, V10×R13/(R12+R13+R14) is inputted via the sixth resistor R16 into the determination control portion 130 (input port AD) and measured thereby. The cathode of a third diode D13 is coupled to a path between the sixth resistor R16 and the input port AD in a manner that anode of the third diode D13 is coupled to the ground voltage G. When the measurement is completed, the third switch SW13 is turned off and a discharge switch SWr is turned on to thereby discharge electric charges of the capacitor C11 via the fifth resistor R15.
Next, the determination control portion 130 charges the capacitor C11 in a state that the one end of the capacitor C11 is grounded via the fourth resistor R14 and measures the voltage set to the capacitor C11. To be more concrete, firstly the determination control portion 130 turns the first switch SW11 and the fourth switch SW14 on. In response to this turning-on operation, there is formed a path from the ground voltage G to the ground voltage G via a negative electrode side grounding resistor RLn, the power supply V, the first switch SW11, the first diode D11, the first resistor R11, the capacitor C11, the fourth switch SW14 and the fourth resistor R14. In this case, a charge voltage VC11 (negative electrode side grounding resistor voltage) is set to the capacitor C11. Then, when the first switch SW11 is turned off and the third switch SW13 is turned on, in the similar manner to the aforesaid case, a divided voltage of the charge voltage VC11 set to the capacitor C11, that is, VC11×R13/(R12+R13+R14) is inputted via the sixth resistor R16 into the determination control portion 130 and measured thereby. When the measurement is completed, the third switch SW13 is turned off and the discharge switch SWr is turned on to thereby discharge electric charges of the capacitor C11 via the fifth resistor R15.
Next, the determination control portion 130 turns the second switch SW12 and the third switch SW13 on. In response to this turning-on operation, there is formed a path from the ground voltage G to the ground voltage G via the third resistor R13, the third switch SW13, the first diode D11, the first resistor R11, the capacitor C11, the second switch SW12, the power supply V and a positive electrode side grounding resistor RLp. In this case, a charge voltage VC12 (positive electrode side grounding resistor voltage) is set to the capacitor C11. Then, when the second switch SW12 is turned off and the fourth switch SW14 is turned on, in the similar manner to the aforesaid case, a divided voltage of the charge voltage VC12 set to the capacitor C12, that is, VC12×R13/(R12+R13+R14) is inputted via the sixth resistor R16 into the determination control portion 130 and measured thereby. When the measurement is completed, the third switch SW13 is turned off and the discharge switch SWr is turned on to thereby discharge electric charges of the capacitor C11 via the fifth resistor R15.
Succeedingly, the determination control portion 130 performs the insulation resister conversion based on a calculation expression (VC11+VC12)/V10 to thereby detect the state of the grounding resistor RL with reference to a predetermined table. When the grounding resistor RL thus detected is equal to or smaller than a predetermined threshold value RLy, the determination control portion 130 determines that the insulation property is degraded and so outputs a predetermined alarm.
The insulation resistance value RLy acting as the threshold value of the positive electrode side grounding resistor RLp and the negative electrode side grounding resistor RLn is required to have the highest detection accuracy. Peripheral circuit constants and respective charge time periods are set so that the charge voltages VC11, VC12 at the time of being determined as the insulation resistance value RLy become equal to the high voltage V10. In recent years, the configuration of employing a ceramic capacitor as the capacitor C in order to miniaturize the insulation measurement circuit 10 has been sometimes employed. However, in this case, it is necessary to take the influence of the DC bias characteristics into consideration. A coefficient relating to the influence of such the characteristics is set to be α and a coefficient relating to the influence of the variations of the peripheral circuit is set to be β. The insulation resistance value is obtained by using an insulation resistance value conversion expression shown in the following expression (A1). In the case of obtaining the insulation resistance value at the time where the negative electrode side grounding resistor RLn is degraded and becomes the predetermined, since the peripheral circuit constants and the respective charge time periods are set so that the charge voltage VC11 becomes equal to the high voltage V10, the influences (coefficients α) on the charge voltage VC11 and the high voltage V10 due to the DC bias characteristics coincide to each other. Thus, the insulation resistance value conversion expression as to the threshold value Rly can be shown in the following expression (A1).
                              Insulation          ⁢                                          ⁢          resistance          ⁢                                          ⁢          value          ⁢                                          ⁢          conversation          ⁢                                          ⁢          expression                =                  (                                                    (                                                      VC                    ⁢                                                                                  ⁢                    11                    ×                    α                    ×                    β                                    +                                      VC                    ⁢                                                                                  ⁢                    12                    ×                                          α                      ′                                        ×                    β                                                  )                            /                              (                                  VC                  ⁢                                                                          ⁢                  10                  ×                  α                  ×                  β                                )                                      =                                          VC                ⁢                                                                  ⁢                11                ×                α                ×                                  β                  /                                      (                                          VC                      ⁢                                                                                          ⁢                      10                      ×                      α                      ×                      β                                        )                                                              =                              VC                ⁢                                                                  ⁢                                  11                  /                  VC                                ⁢                                                                  ⁢                10                                                                        (                  A          ⁢                                          ⁢          1                )            
That is, α and β at each of the numerator and the denominator are cancelled. In other words, since the influences due to the variations of the characteristics of the capacitor C11 and the peripheral circuits can be excluded, the measurement can be made accurately. In a range that the accuracy is not required where the insulation resistance value does not coincide with the threshold value Rly, the variation of the ceramic capacitor (capacitor C11) due to the DC bias characteristics is corrected by using a software in a manner of approximating the general DC bias characteristics. By employing such the technique, the measuring time period is shortened and such an influence on the ceramic capacitor due to the DC bias characteristics that the capacitance value of the capacitor reduces in accordance with the increase of an applied voltage is removed.
As shown in FIG. 2, unlike a film capacitor, the ceramic capacitor has the aforesaid DC bias characteristics. A steady line represents theoretical values. The individual ceramic capacitors have variances and errors in their characteristics as shown by a dotted line (a measurement value example 1) and a two-dot chain line (a measurement value example 2) in the figure. Thus, since the directions and widths of the variances with respect to the theoretical values are not constant depending on the applied voltage, it is difficult to improve the detection accuracy as the entirety of the apparatus.
To be concrete, in the case of measuring the grounding resistor RL, the charge resistor for the high voltage V10 is R11, whilst the charge resistor for the charge voltages VC11, VC12 is R11+RL (RLp or RLn). In this case, the peripheral circuit constants and respective charge time periods are set so that the charge voltages VC11, VC12 become equal to the high voltage V10 when the grounding resistor RL is the aforesaid threshold value Rly. The charge time period of the high voltage V10 was set to be shorter than the charge time period of the charge voltages VC11, VC12. The DC bias characteristics of the capacitor C11 changes depending on the voltage applying time period as well as the change of the applied voltage. As a result, the influence of the DC bias characteristics at the time of measuring the high voltage V10 does not coincide with the influence of the DC bias characteristics at the time of measuring the charge voltages VC11, VC12. Thus, there arise a problem that the influence of the coefficient α in the aforesaid expression (A1) can not be excluded. Further, since the degree of the change differs depending on the individual devices (ceramic capacitors), there also arises a problem that the correction can not be performed completely by the software.
Further, there arises a problem that the cost of the apparatus becomes very high when the requirement of the accuracy with respect to the aforesaid variances of the ceramic capacitor is made restrict. In particular, in recent years, since there are supposed cases which are insufficient in the accuracy that have been allowed conventionally, a new technique has been demanded capable of being employed in view of the accuracy and cost.